The manufacturing of intaglio printing plates for the production of securities by direct laser engraving is already know in the art.
International application No. WO 97/48555 A1 for instance discloses a method for the direct engraving of a metallic printing plate medium wherein a precision engraving tool, such as a laser engraving tool, is guided in such a way as to follow a determined tool track located within a desired contour and with a desired penetration depth to remove material from the printing plate medium within the desired contour at the desired depth. This engraving process is essentially vectorial in that the laser engraving tool follows a determined tool trajectory corresponding to the intaglio printing patterns to be engraved.
European patent application No. EP 1 334 822 A2 discloses a method and installation for the direct laser engraving of intaglio printing plates or cylinders wherein engraving is performed by means of a laser beam generated by a pulsed Nd-YAG laser device. A laser-engravable printing plate medium is mounted onto a motor-driven platform capable of moving along two Cartesian axes and which is controlled by a computer, which computer also controls operation and actuation of the pulsed laser device. The pulsed laser device is mounted so as to be vertically moveable and the height thereof with respect to the platform is adjustable by means of a motor controlled by the computer so as to correct and adjust focusing of the laser beam onto the plate to be engraved. An optical system with galvanometric motors is used to guide the laser beam on a focal lens that concentrates the laser beam on a desired point within a determined engraving area that covers only a limited part of the whole printing plate surface. The pulsed laser device is specifically designed to generate pulses whose power is largely higher than the power of ordinary continuously-operated laser devices. One disadvantage of this solution resides in the fact that the engraving process involves a repetitive local processing to engrave the whole area of the printing plate, an engraving area of the order of 100 mm×100 mm being treated at a time. An optical system with appropriate focal length is provided to process each desired location within the engraving area. The angle of incidence of the laser beam with respect to the surface of the printing plate thus varies according to the position of the laser beam with respect to the locations of the engraving area being engraved, thereby potentially affecting engraving uniformity. Such processing moreover requires a very high accuracy so that no overlaps or gaps appear between adjacent engraving areas.
International application No. WO 2006/045128 A1 discloses a direct laser engraving system for the production of intaglio printing plates that follows a similar approach to the one described in the above-mentioned European patent application No. EP 1 334 822 A2, i.e. the laser-engravable printing medium is similarly mounted onto a movable x-y platform, while a laser device is controlled to engrave a limited area of the printing plate surface. Further details about this direct laser engraving system may be found in the article of Messrs. Harald Deinhammer, Daniel Schwarzbach, Rudolf Kefeder and Peter Fajmann entitled “The implication of direct laser engraved intaglio printing plates on banknote security”, Proceedings of SPIE, Vol. 6075, 2006, 607503-1 to 607503-11 (hereinafter referred to as Deinhammer2006). The engraving area of this other system is of about 70 mm×70 mm, which implies that only a limited part of the intaglio printing patterns might be engraved at a time and that the engraving process must be repeated with high accuracy so that no visible overlaps or gaps are formed between adjacent engraving areas. As disclosed in International application No. WO 2006/045128 A1, this implies performing a calibration operation at regular intervals which is cumbersome and time-consuming.
A further disadvantage of the direct laser engraving system of International application No. WO 2006/045128 A1 again resides in the fact that the angle of incidence of the laser beam with respect to the printing plate surface varies according to the position of the laser beam with respect to the locations of the engraving area being treated, which potentially leads to engraving non-uniformities which shall be avoided.
Yet another disadvantage of this direct laser engraving system (which also characterizes the approach disclosed in International application No. WO 97/48555 A1) resides in the fact that the data used to control the laser engraving device is essentially based on vectorized graphic data. This implies that engraving times will increase with design complexity as discussed in Deinhammer2006.
Further discussion regarding the direct laser engraving of intaglio printing plates might further be found in International application No. WO 2005/002869 A1 and the related article of Messrs. Harald Deinhammer, Franz Loos, Daniel Schwarzbach and Peter Fajmann entitled “Direct Laser Engraving of Intaglio Printing Plates”, Proceedings of SPIE, Vol. 5310, 2004, pp. 184-193 (hereinafter referred to as Deinhammer2004). According to International application No. WO 2005/002869 A1, the printing plate, which is made of or has an outer layer consisting of brass or an alloy thereof, is engraved directly by laser and, once fully engraved, is subjected to a cleaning process to remove the melted residues of the laser engraving process, which cleaning process involves a chemical treatment in a chemical bath of acidic solution, before being ultimately polished and chromed. A dry-ice pre-cleaning of the surface of the engraved printing plate by spraying of solid carbon-dioxide pellets may be performed prior to the chemical treatment of the engraved plate (see again Deinhammer2006). The proposed chemical post-engraving treatment of the engraved printing plate, as well as the dry-ice pre-cleaning, is rather aggressive and can lead to a degradation of the desired intaglio engravings if it is not controlled and carried out properly. The post-engraving processing with dry-ice pre-cleaning and treatment with acidic solution are furthermore prejudicial as they could lead to engraving non-uniformities and lack of repeatability, namely differences between two intaglio printing plates produced from a same design.
International application No. WO 03/103962 A1 in the name of the present Applicant, which is incorporated herein by reference in its entirety, discloses a different approach to the manufacture of engraved plates for the production of intaglio printing plates. While the intaglio design to be engraved is conceived on a computer assisted design system using vectorized graphics and/or bitmap graphics, the final intaglio design is converted into a so-called depth-map that basically consists of three-dimensional pixel data representative of the intaglio printing patterns to engrave and where the “intensity” of each pixel is representative of the depth to be engraved at the corresponding location of the printing plate. A laser engraving device is then controlled based on this depth-map to perform a series of successive elementary engraving steps corresponding to each pixel of the depth-map. A typical resolution of engraving can be as high as 8000 dpi, which amounts to a distance between two successive elementary engraving steps of approximately 3 microns. A printing plate can be thus engraved by a laser beam based on the generated depth-map to produce engravings into the printing plate surface.
A substantial advantage of this approach resides in the fact that the various design elements forming the whole intaglio design to be engraved in the printing plate are not engraved individually, but rather all at once pixel by pixel. Furthermore, and in contrast to the above-mentioned approaches, the engraving times are independent of the complexity of the intaglio design and depend only on the maximum depth of the design to be engraved. This pixel by pixel approach further translates into greater flexibility and control regarding the engraving profile and the shape thereof.
The engraving method disclosed in International application No. WO 03/103962 A1 can in particular be put into practice to engrave highly complex intaglio printing patterns such as disclosed in International applications Nos. WO 2005/090090 A1 and WO 2007/119203 A1, also in the name of the present Applicant. The pixel-by-pixel engraving principle disclosed in WO 03/103962 A1 is particularly suitable to engrave the patterns discussed in International application No. WO 2007/119203 A1 which exhibit a high complexity and density of elements, and this with reasonably low engraving times. Illustrative intaglio-printed patterns are shown in FIGS. 1 and 2, which intaglio-printed patterns are analogous to those found on most intaglio-printed banknotes. FIG. 1 is an enlarged view of an eye portion of a portrait that can be found in International application No. WO 03/103962 A1, while FIG. 2 is an enlarged view of an eye portion of another portrait according to the teaching of International application No. WO 2007/119203 A1 wherein typical curvilinear intaglio-printed patterns are interlaced with micro-letters that are dimensionally modulated to produce variations in tones. These intaglio-printed patterns are typically formed of a complex arrangement of dimensionally modulated curvilinear elements that produce the desired halftones of the pictorial representation that one wishes to create on the security paper. The curvilinear elements typically have a line width of the order of 10 microns and more.
As discussed in International application No. WO 03/103962 A1, the pixel-by-pixel engraving method can either be put into practice to engrave a precursor of an intaglio printing plate, such as a polymer plate, which precursor is then used to produce any number of identical printing plates by a galvanic replication process, or to directly engrave a metallic plate that is ultimately used as the intaglio printing plate, typically after chroming of the surface thereof. Advantageously, the engraving process is carried out by mounting the laser-engravable printing medium on the circumference of a rotating cylinder and moving the laser engraving device in a direction parallel to the axis of the cylinder.
The laser engraving of polymer precursors of intaglio printing plates as taught in International application No. WO 03/103962 A1 has been successfully put into practice by the Applicant which has now sold a number of Computer to Intaglio Plate®, or CTiP®, systems to the banknote printing industry worldwide. Such CTiP® systems are now used by a substantial number of banknote pre-press centres around the world for the manufacture of intaglio printing plates for the production of banknotes and other security papers.
The laser engraving of polymer precursors of intaglio printing plates is advantageous over the direct laser engraving of printing plates in that engraving of a polymer precursor can be performed in a single pass and with a high engraving quality. Engraving of polymer precursors is accordingly particularly suited to the manufacture of high quality, high resolution intaglio printing plates for the production of banknotes.
Another advantage of the laser engraving of polymer precursors of intaglio printing plates resides in the fact that such precursors, once engraved, can be used to produce a so-called “Alto” by a galvanic process, which Alto can be used to produce a master (or original) printing plate used to replicate multiple absolutely identical intaglio printing plates or as “master Alto” to directly produce such multiple absolutely identical intaglio printing plates. Since the galvanic process is a very stable and homogenous process, the likelihood that differences occur between multiple printing plates which have been replicated from a same master is almost inexistent. Furthermore, the same printing plate materials as conventionally used in the industry can be exploited without this bearing any impact on the actual work of the printer operating the printing press.
Direct laser engraving of intaglio printing plates however has an advantage in terms of environmental requirements in that it enables to circumvent the galvanic process which makes use of environmentally unfriendly chemical agents.
Tests carried out by the Applicant have demonstrated that the direct laser engraving of printing plate mediums, especially metallic mediums, leads to the formation of residues which need to be removed to ensure proper engraving and printing quality. The amount of residues is directly dependent on the depth of the engravings and therefore increases with engraving depth. These amounts of residues become particularly critical to remove as soon as one wishes to form relatively deep engravings in the printing plate medium, especially engravings having a depth exceeding 50 microns (and even more critically above 100 microns).
While the particular choice of the printing plate material impacts on the formation of melted residues, such formation is unavoidable in the context of the direct laser engraving of intaglio printing plates and care should be taken to remove these residues while avoiding to degrade the quality of the desired engravings. This is again very critical when it comes to engraving deep intaglio printing patterns as substantial amounts of residues are produced that become difficult to remove.
This constitutes one of the reasons why it is proposed in Deinhammer2006 to perform a pre-cleaning operation of the surface of the engraving printing plate by spraying dry-ice pellets of carbon dioxide at low temperature (of the order of −80° C.) on the surface of the plate and then to subject the pre-cleaned printing plate to a chemical treatment in an acidic solution. As already mentioned, such operations are potentially prejudicial as they could lead to engraving non-uniformities. These cleaning operations can furthermore only be carried out once the printing plate has been fully engraved.
It has also been noticed that while the energy of the laser beam is well absorbed and converted into thermal energy in the first few micrometers of the material of the engraved printing plate medium, resulting in a local melting and evaporation of the material, the thermal energy is no longer directed and is spreading evenly in all direction into the material lower in the printing plate medium. As a result, higher laser energy do not only create deeper engravings, but also increased line widths (see again Deinhammer2004).
There is therefore a need for an improved method and system for the manufacture of intaglio printing plates wherein a laser beam is used to engrave intaglio printing patterns directly into the surface of a laser-engravable, especially metallic, printing plate medium.